Rhesus monkeys (Macaca mulatta) reared during the first year of life without social contact develop persistent stereotyped movements, self-directed behaviors, and psychosocial abnormalities, but neurobiological mechanisms underlying the behaviors of socially deprived (SD) monkeys are unknown. Monkeys were reared in total social deprivation for the first 9 months of life; control monkeys were reared socially (SR) with mothers and peers. Subjects were killed at 19-24 yr of age. Because the behaviors of SD monkeys are reminiscent of changes in striatal or amygdalar function, we used immunocytochemistry for substance P (SP), leutine-enkephalin (LENK), somatostatin, calbindin, and tyrosine hydroxylase (TH) to evaluate qualitatively and quantitatively patterns of neurotransmitter marker immunoreactivity within subcortical regions. In SD monkeys, the chemoarchitecture of the striatum was altered. Neuronal cell bodies and processes immunoreactive for SP and LENK were depleted markedly in patch (striosome) and matrix regions of the caudate nucleus and putamen; the average density of SP-immunoreactive neurons was reduced 58% relative to SR monkeys. Calbindin and TH immunoreactivities were diminished in the matrix of caudate and putamen of SD monkeys. TH-immunoreactive neurons, but not cresyl violet-stained neurons, in the substantia nigra pars compacta were decreased (43%) in SD monkeys. Peptide-immunoreactive terminals were reduced in the globus pallidus and substantia nigra in SD monkeys. The nucleus accumbens was the least affected of striatal regions. Striatal somatostatin immunoreactivity was qualitatively and quantitatively similar in SD and SR monkeys. Several regions, for example, bed nucleus of the stria terminalis, amygdala, and basal forebrain magnocellular complex, that were in the same sections and are enriched in these markers did not appear altered in SD monkeys, suggesting a regional specificity for vulnerability. The altered chemoarchitecture of some basal ganglia regions in adult monkeys that experienced social deprivation as infants suggests that the postnatal maturation of neurotransmitter phenotypes in some structures is influenced by social environment. Abnormal motor and psychosocial behaviors resulting from this form of social/sensory deprivation may result from alterations in peptidergic and dopaminergic systems within the basal ganglia.
Martin, L. J., Spicer, D. M., Lewis, M. H., Gluck, J. P., & Cork, L. C. (1991). Social deprivation of infant rhesus monkeys alters the chemoarchitecture of the brain: I. Subcortical regions. The Journal of neuroscience, 11(11), 3344-3358.